1. Field of the Invention
This invention relates to a vehicle steering control system.
2. Description of the Related Art
Various technologies have recently been proposed for using a CCD (Charge-Coupled Device) camera or the like mounted on a vehicle, which takes an image of the surface of an expressway or any other road on which the vehicle is traveling and detects white lines (lines painted on the road and defining a lane boundary) from the obtained image and based on this, controlling steering assist torque of the vehicle such that the vehicle runs along the lane. In this kind of steering assistance control, the system determines an error (deviation) between a desired position set on a noticed point on the lane ahead of the vehicle and a point to which the vehicle is predicted to reach, calculates information relating to the structure of the lane such as its curvature, and based thereon, determines the required steering assist torque.
In order to determine the point to which the vehicle is predicted to reach, two methods are widely used, the so-called "first prediction method" and the "second prediction method". In the first prediction method, the point is calculated based on the vehicle heading angle at a current vehicle position on the lane from the lane information obtained from the image. In the second prediction method, the point is calculated based on the yaw rate (angular velocity of yaw) around the vertical axis (the axis of gravity) detected by a yaw rate sensor mounted on the vehicle. The assignee proposed the latter in Japanese Laid-Open Patent Application No. Hei 5 (1993)-197423.
However, as regards the first prediction method, since this uses the lane information obtained from the image, an accidental error grows with increasing distance, in other words, an accidental error grows as the point is set at a distance farther from the vehicle. In addition, the accidental error also grows due to a change of posture of the vehicle (on which the camera is mounted), such as pitching or rolling, bumps or undulations of the lane, etc.
As a result, when conducting a feedback control such that the error (deviation) between the predicted point (after a predicted period of time) and a desired position obtained from the image decreased, it is greatly affected by the accidental error involved in the image. This sometimes makes the steering assist torque to be calculated inadequately, thereby degrading the accuracy of the steering assistance control.
As regards the second prediction method, the yaw rate is usually detected using a yaw rate sensor mounted on the vehicle. However, since the output of the yaw rate sensor involves accidental errors due to offset or drift, the predicted point determined from the yaw rate sensor output is not free from accidental errors, thereby rendering similar problems to occur.